js判断子窗体是否关闭的方法

function InsideMessageAdd() {
 //window.open()得到子窗体
 tip = OpenDialog('UploadFile.html?tp=nbtz', 'msg', 650, 180);
 //启动timer，判断子窗体是否关闭
 timer = window.setInterval("IfWindowClosed()", 500);
}
var timer;
var tip;
function IfWindowClosed() {
 //判断子窗体是否关闭
 if (tip.closed == true) {
  alert("close");
  window.clearInterval(timer);
 }
}